1. Field of the Invention
This invention relates to an improvement of an actuator for operating a manipulator (artificial hand) or the like for treating or handling radioactive substances surrounded by shield walls or hot cells for protecting from contamination of radiativity in storing, replacing and distributing the radioactive substances and for other purposes such as experiments and working.
2. Description of the Prior Art
Various kinds of actuators have been proposed for manipulators provided for the above mentioned hot cells. Among these proposals, electric systems including electric motors require reduction gears to increase total weight and make their construction complicated, and moreover unavoidably generate sparks in operation, which make impossible to use the electric systems at locations requiring explosion-proof. Hydraulic systems have disadvantages in that hydraulic units are expensive and heavy and oils used in the units tend to contaminate themselves, other parts and rooms.
Pneumatic systems are preferably used in case particularly requiring explosion-proof. However, pneumatic actuators in general do not provide smooth movements due to sliding resistance between cylinders and pistons which are worse still usually made of steel, so that the self weights of their operating or movable parts are too large in comparison with their operating forces, resulting in lower operating accuracy.
With these hitherto used driving systems, outputs are usually constant during displacement, so that positioning in strokes cannot be effected smoothly. As a result, arms accidentally collide against objects to damage them and, therefore, these systems require great skill.
On the other hand, air bag type actuators have been known. In such actuators, operating forces are obtained from axial contracting forces of the air bags due to enlargement of their diameters when applying controlling pressure to them. In addition to the light weight of the bags themselves, they have advantages of no risk of air leakage and no problems due to friction because of no sliding parts. The amount of air enclosed in the bag for the application of the controlling pressure increases due to the expansion of the bag in comparison with the pneumatic cylinders and is dissipated into the atmosphere when it is released. It is disadvantageous to consume a great amount of air for operating. Moreover, the above controlling pressure acts upon all over inner surfaces of cavity walls in the air bag, so that the effective axial contracting forces are reduced by resistance representative of a product of the control pressure and cross-sectional area perpendicular to the axis of the bag subjected to the control pressure at least during an initial period of its operation.
A pneumatic actuator of such an air bag type for example as shown in FIG. 1 has been publicly known disclosed in Japanese Patent Application Publication No. 40,378/77.
The pneumatic actuator as shown in FIG. 1 comprises a tubular body 1, a reinforcing braided structure 2 externally thereon, closure members 3 at both ends and a clamp sleeve 4. The tubular body 1 is preferably made of a rubber or rubber-like elastomer which is air-impermeable and flexible. However, other materials equivalent thereto, for example, various kinds of plastics may be used for this purpose. The reinforcing braided structure 2 reinforced by cords is somewhat similar to, for example, those conventional in pressure-resistant rubber hoses, whose braided angles are approximate to what is called an angle of repose (54.degree.44'). With the braided structure of the pneumatic actuator, initial braided angles .theta..sub.o are preferably of the order of 20.degree. in order to obtain the above angle of repose when the reinforcing structure is expanded to the maximum diameter due to inner pressure filled or supplied in the tubular body 1. In this case, conditions of use are determined so as to permit a strain under normally used conditions to be of the order of 0.3.
The reinforcing cords used in the braided structure 2 are organic or inorganic high tensile fibers, preferably, for example, twisted or nontwisted filament bundles of aromatic polyamide fibers (trade name, KEVLAR) or very fine metal wires.
With a considerably small angle of the initial braided angle .theta..sub.o such as 20.degree., it is not necesarily easy to braid the outer circumference of the tubular body 1. For example, however, a braided structure obtained by a conventional hose braiding machine is stretched in its axial direction to be commensurate with the above initial value and is then fitted on the tubular body 1 under the stretched condition, thereby obtaining the desired braided structure. In this case, a suitable adhesive may be applied the outer circumference of the tubular body 1. An outer sheath of a weatherproof or injury-protective film may be preferably provided on the braided structure 2.
Each closure member 3 comprises a nipple 5 adapted to be closely fitted in each end of the tubular body 1 preferably with an adhesive for sealing the tubular body from the atmosphere, a flange 6 for positioning the closure member relative to the tubular body, and an eye or yoke 7 having an aperture for a connecting pin (not shown). The nipple 5 is preferably provided on its outer periphery with annular protrusions 8 each having a steep taper surface toward the eye 7 and a gentle taper surface in an opposite direction for preventing the nipple 5 from being removed. One of the closure members 3 is formed at least on one side with a connecting aperture 11 communicating with an inner cavity 10 of the tubular body 1 through an aperture 9 formed in the nipple 5 in its axial direction. A fitting 12 is fitted in the connecting aperture 11 of the closure member 3.
Each clamp sleeve 4 is a cylindrical metal member engaging the flange 6 so as to cover the end outer circumference of the tubular body 1 and having a flare 13. The clamp sleeve 4 is partially pressed toward the nipple 5 in its radial directions to sealingly unite the closure member 3 with the tubular body 1. Reference numeral 14 in FIG. 1 denotes axial depressions caused by a calking tool in its process.
To the fitting 12 is connected an operating pressure source, for example, an air compressor through a line having a three-way valve (not shown).
When a controlled pressure P is applied into the inner cavity 10 of the tubular body 1 through the fitting 12, the braided structure 2 is expanded from the position shown in solid lines to that shown in phantom lines in FIG. 1 to enlarge the initial braided angles .theta..sub.o to .theta..sub.x or in a pantograph movement of the reinforcing cords of the braided structure 2 so as to cause an enlargement of the diameter of the tubular body 1 and a contraction in its axial direction caused thereby. A force F of the contraction is given by the following equation (1) ##EQU1##
On the other hand, when the controlled pressure in the cavity 10 of the tubular body 1 is released through the three-way valve into the atmosphere, the tubular body 1 of course regains its length with decrease of the braided angle .theta..sub.x.
It is therefore understood that such a pneumatic actuator can bring about bending and extending movement or articulate movements between two pivotally connected or articulated operating arms to which the eyes or yokes 7 of the closure members 3 of the pneumatic actuator are connected by means of pins.
In general, the reinforcing braided structure 2 includes the reinforcing cords embedded in the proximity of an outer surface of the tubular body of a rubber-like elastomer so as to be securely united with each other. With this arrangement, however, the movement of the reinforcing cords in articulated movements of arms is restrained by the rubber-like elastomer to reduce the force of contraction. It is, therefore, preferable to separate the reinforcing cords from the tubular body so as to permit free movements of the cords. However, such a construction permitting the reinforcing cords to separate from the tubular body encounters another problem. Each reinforcing cord consists of a number of fine filaments 15 having diameters of 0.02-0.03 mm which are twisted or nontwisted as shown in FIG. 2. Upon expansion of the elastic tubular body, the fine filaments are likely to bite into the tubular body due to the greatly changed braided angles of the cords. Accordingly, the filaments injure the surface of the tubular body, as if they were very sharp knives, resulting in breaking down of the tubular body.
Even if the pneumatic actuator includes the tubular body with reinforcing cords embedded therein so as not to separate therefrom, there is in general a tendency of the cords to penetrate into the elastic tubular body to reduce the service life of the tubular body and hence the actuator, because its diameter increases to 2-3 times of its original diameter and its axial length contracts to 20-40% of its original length so as to cause maximum stresses acting upon the reinforcing cords.
In general, with the above conventional construction of the actuators, there is a relation between the initial braided angles .theta..sub.o, changed braided angles .theta..sub.x upon contraction and contractive strains .epsilon.. EQU .epsilon.=(Cos .theta..sub.o -cos .theta..sub.x) (2)
Moreover, there is a relation as set forth in the following equation (3) between original diameters D.sub.o of the braided structure 2 and contracted diameters D and a relation as the following equation (4) between the contractive forces F and the other parameters including controlled pressure P. ##EQU2##
These relations hold true at approximate center zones of the actuators. However, these relations do not hold true at both the ends of the actuators, because the diameters at the ends are restrained from changing. As shown in FIG. 4 schematically illustrating an expanded braided structure 2 from an original condition shown in FIG. 3, the ends of the braided structure 2 are prevented from expanding to cause unnatural strains of the structure which would give rise to fatigue failure of the structure and obstruct the occurrence of the expected contractive force.
A solid line A in FIG. 5 illustrates a relation between the contractive force F and the strain .epsilon. obtained from the equation (4) when the controlled pressure P is constant. A broken line B in FIG. 5 was obtained by actually measuring contractive forces and strains with actuators of the prior art having constant diameters of the brained structures and constant brained angles. The broken line B is fairly different from the theoretical line A due to the restraint of both the ends of the brained structure 2. A dot-and-dash line C and a two-dot line I will be explained latter.
As above described, the actuator is radially expansible and simultaneously axially contractible with the aid of controlled pressure applied therein has the various advantages of light weight as a whole, smooth movements in operation and reliable performance of positioning which are not obtained by the systems using electric motors or hydraulic piston and cylinder assemblies. Various applications are considered utilizing these superior characteristics of the expansible and contractible actuator.
FIG. 6 illustrates a linear driving system actuated with pneumatic pressure including a servo-valve having a torque motor 21, nozzles 22 and 23, a flapper 24 and throttle valves 25 and 26, and two elastic actuators 27 and 28 arranged in series with aligned axes.
With this arrangement, for operation, the torque motor 21 is energized and the flapper 24 is deflected for example in a direction shown by an arrow D, so that a clearance between the nozzle 22 and the flapper 24 becomes smaller and a clearance between the nozzle 23 and the flapper 24 becomes correspondingly larger. Accordingly, a back pressure on a side of the nozzle 22 approaches the supplied pressure of the air and a back pressure on a side of the nozzle 23 approaches the atmospheric pressure, respectively. As a result, the pressurized air is supplied into the actuator 27, while the pressurized air in the actuator 28 is exhausted, so that a mass point 29 is driven in a direction shown by an arrow E. When the flapper 24 is deflected in the direction opposite to the arrow D, it should be understood that the mass point 29 is driven in the direction opposite to the arrow E.
There has been proposed a driving system similar to the above linear driving system, wherein back pressure of nozzles is changed to move a spool of a guide valve to change air flow.
However, the above systems are expensive to manufacture due to high cost torque motors used therein, complicated in construction and slow in response. Under a steady condition, moreover, the pressurized air leaks at nozzles to cause noise and to increase consumption of the pressurized air. In addition, exact positioning cannot be easily effected with these systems.